How to make new plugin setting checkbox default as checked

If you check the checkbox and save the plugin options it will add option to the {prefix}_options table in the database. To make a checkbox checked you need to update_option('option_id', 'value' ) for that checkbox on plugin activation. to do that you need to register plugin activation hook. Place the below code in the main plugin file.

register_activation_hook( __FILE__, 'am_plugin_activate' );
function am_plugin_activate() {
    // plugin activation code here...
    update_option('checkbox_id', true );
}

this will make the checkbox checked. to make the checkbox checked on plugin update you need to hook a function on upgrader_process_complete action hook. e.g;

add_action( 'upgrader_process_complete', 'am_plugin_upgrate',10, 2);
function am_plugin_upgrate( $upgrader_object, $options ) {
    // plugin update code here....
}

for reference please take a look at upgrader_process_complete, Discussion Plugin Activation Hooks and Plugin activation hook discussion